In the eye, the final stage of image processing in the retina is performed by retinal ganglion cells. The axons of the retinal ganglion cells project out of the eye to the optic nerve. Glaucoma, which produces irreversible blindness if not treated early enough, destroys these ganglion cells. A classical diagnostic feature of glaucoma is the occurrence of a scotoma (the disappearance of a portion of the peripheral visual field). Unfortunately, by the time a scotoma is detected, the disease has reached a stage where treatment is able to do little more than possibly prevent the occurrence of further blindness. Another classical epidemiological feature of glaucoma is the "cupping" of the optic disc, or papilla, which to some extent reflects the loss of ganglion cell axons.
One approach that has been used to assist in the diagnosis of glaucoma before scotomas have developed is to test the intraocular tension of patients. Tests of intraocular tension, however, often involve the use of drugs, are time consuming, and can be unpleasant for the patient. In some cases, potentially damaging elevated tensions occur only transiently and at unpredictable times during the day. Such transient high tensions may not be detected. Moreover, up to half of all glaucoma patients do not exhibit sustained ocular tensions above 21 mm of mercury (these patients are described as having "low-tension" or "normal-tension" glaucoma). Thus, testing the intraocular tension of a person is not a reliable method for the early detection of glaucoma.
Other proposals for the early detection of glaucomatous retinal damage have involved the assessment of color vision defects. Simple tests of color vision defects, however, have shown a lack of correlation between the defects noted and the presence of optic disk cupping. More complex tests of color vision defects, involving anomaloscopy, are too difficult for clinical use. Moreover, those tests cannot differentiate between color vision defects caused by glaucoma and color deficits caused by other neurological disorders, such as amblyopia and optic neuritis. In addition, it has been reported that up to 25 per cent of patients who have glaucomatous scotoma show no color defects. Thus, assessment of color vision deficits is not a reliable method of detecting glaucoma in its early stages, even if it should become practical to perform detailed color vision tests clinically.
Another problem with psychophysical measurements of visual performance is that the results of such measurements vary significantly among subjects with normal vision. This natural variance reduces the ability to use such measurements to distinguish normal subjects from those with early glaucomatous damage. There has been a long-felt need, therefore, for a method of measuring visual performance which is less subjective than the psychophysical tests that have been carried out hitherto.